Method for determining free phosphoric acid content in trichlorethylene-based phosphatizing baths



United States Patent US. Cl. 23-230 4 Claims ABSTRACT OF THE DISCLOSURE A process for determining the free phosphoric acid content in a trichlorethylene-based anhydrous phosphatizing bath by a simple and efficient titration procedure on a sample obtained by partition using extraction by mutually insoluble solvents.

Background of the invention Phosphate coatings are widely employed to protect metals against corrosion and to improve paint adherence to metals. Such coatings are applied to cold-rolled and galvanized steels and to other metallic substrates. This is commonly known as phosphatizing. Baths producing such coatings are typically based on water or halogenated hydrocarbons.

Typical halogenated hydrocarbon-based phosphatizing baths contain orthophosphoric acid as the active ingredient blended into trichlorethylene by alkyl acid phosphates and/or aliphatic alcohols as shown, for example, in US. Patent No. 2,789,070 and British Patent No. 891,390. Metal articles are phosphatized generally by being immersed in or sprayed with phosphatizing bath. In commercial operations articles are successfully processed by contact with the bath. As would be expected, the composition of the phosphatizing bath changes with the treatment of each article. When using the same immersion time for each article, as is the case in commercial operations, this change in the composition of the bath results in the forming of thinner and thinner phosphate coatings on each successive metal article. During the phosphatizing of such metal articles, at least two chemical reactions take place which reduce the phosphoric acid concentration of the bath. First, phosphoric acid reacts with the surface of the metal articles being treated. This, of course, is the desired reaction. The rate at which this reaction proceeds at any time is primarily dependent upon the work load being processed, the concentration of phosphoric acid in the bath, temperature, and contact time. Secondly, phosphoric acid reacts with the aliphatic alcohol of the bath to form alkyl phosphates, particularly to form monoalkyl acid phosphates and dialkyl acid phosphates. This side reaction proceeds more rapidly as the work loads become heavier and as the weight of the coating being applied increases. Not only do these two reactions reduce the phosphoric acid concentration of the bath, but the alkyl phosphates interfere with the unreacted phosphoric acid remaining in the bath so that a higher concentration of phosphoric acid is needed than if the alkyl phosphates were not present. Accordingly, phosphoric acid must be added to replace the acid which has reacted to form the surface coating and also to compensate for the interfering influence of the alkyl phosphates in order to maintain a constant rate of phosphate coating formation.

As more and more metal articles are phosphatized in a given phosphatizing bath, more phosphoric acid is added and more alkyl phosphates are formed. The interfering influence of the alkyl phosphates increases directly Patented Aug. 5, 1969 with the alkyl phosphate concentration of the bath. Eventually, the interfering influence becomes so great that it is no longer economically feasible to add additional phosphoric acid to the bath. When the composition of the bath reaches this stage, the bath is usually distilled to recover the volatile ingredients in the bath.

Various methods have been developed to determine and regulate the contents of the phosphatizing bath. For example, the acid content of such baths can be measured by titrating a sample of the bath. Unfortunately, this method cannot distinguish between phosphoric acid, monoalkyl acid phosphates and dialkyl acid phosphates. Thus, it is described as a measure of Total Acid. However, since monoand di-alkyl acid phosphates form essentially no phosphate coating, it is important to have a specific measure of phosphoric acid only which is designated as Active Acid.

There are known methods for determining the percentage of each component in mixtures of phosphoric acid, monoand di-alkyl acid phosphates. These are based on titrating the unknown in a mixture of neutral distilled water and a lower aliphatic alcohol such as ethanol or propanol to insure solubilization of the alkyl acid phosphates. The titration is carried out with a sodium hydroxide solution using a pH meter to determine the first and second end points. At the second end point, a constituent is added to precipitate out the phosphates and release the third hydrogen of any phosphoric acid present so that the titration may be continued to a third end point. Examples of the precipitating additives are calcium chloride, barium chloride and silver nitrate. The relationship between the milliliters of sodium hydroxide to the first, second and third end points may be used to indicate the relative amounts of phosphoric acid, monoand di-alkyl acid phosphates. Such analyses are somewhat lengthy, involve an expensive pH meter and require considerable care for accurate results. In addition, their application to trichlorethylene-based phosphatizing baths is complicated by the need to evaporate the bath sample to partial dryness prior to addition of water and alcohol in order to avoid interference of HCl potentially formed from trichlorethylene during the course of the titration.

Attempts have also been made to control the concentration of phosphoric acid in trichlorethylene-based phosphatizing baths by determining the coating characteristics of the bath. A weighing procedure has been used whereby phosphoric acid is added to the bath system as required to maintain, for a given treatment time, a constant phosphate coating weight on the metal articles. This method is time consuming and involves an expensive analytical balance; hence, this is not normally a practical means of controlling phosphoric acid additions.

Thus, a definite need exists for a method of determining the free phosphoric acid content of a trichloroethylenebased phosphatizing bath which is easy to operate and produces accurate results. Preferably, the method for determining free phosphoric acid content can be correlated with a method for controlling phosphoric acid content to provide an efiicient and economic phosphatizing operation which will produce the desired coating characteristics.

Description of the invention This invention relates to a method of determining the free phosphoric acid content of a trichlorethylene-based anhydrous phosphatizing bath. Briefly, the actual free phosphoric acid content is determined by a relatively simple separation and titration procedure. Taking this free phosphoric acid content and subtracting it from the empirical desired free phosphoric acid value, the addi- 3 tional amount of fresh phosphoric acid necessary to achieve the desired free acid content of the bath can be calculated and subsequently added to the bath to control the coating characteristics of said bath.

Accordingly, the process for determining the free phosphoric acid content of a trichlorethylene-based phosphatizing bath comprises (a) obtaining a weighed bath sample, (b) adding to the sample a liquid composition consisting essentially of two immiscible solvents, one of said solvents being a good solvent for alkyl acid phosphates and a relatively poor solvent for phosphoric acid, the other of said solvents being a good solvent for phosphoric acid and a relatively poor solvent for alkyl acid phosphates, said two solvents being substantially insoluble and immiscible in each other and sufficiently different in specific gravity from each other so as to form two separate solution layers, (c) separating the two solution layers and (d) titrating the solution layer containing the phosphoric acid with a standard basic solution to obtain a value which is the measure of the free phosphoric acid in the phosphatizing bath per weight of bath sample. The amount of free acid is termed Active Acid.

This process differs from the prior art chiefly in that it provides a relatively simple method of measuring the free phosphoric acid (Active Acid) as opposed to the titration method of the prior art which measures the Total Acid, i.e. the combined amount of monoalkyl acid phosphate, dialkyl acid phosphate and phosphoric acid in a bath sample, or to the previously discussed complicated three end point titration method of the prior art.

The process of this invention is applicable to any trichlorethylene-based phOsphatiZing bath and is not limited to the baths specifically disclosed herein. A typical trichlorethylene-based phosphatizing bath, may contain 72 to 98 percent by weight of trichlorethylene, 0.1 to 3 percent by weight of phosphoric (preferably ortho-phosphoric) acid and 2 to 25 percent by weight of a blending agent (e.g. a C -C aliphatic alcohol) for the trichlorethylene and phosphoric acid.

Generally, the process for determining the phosphoric acid content of this invention can be performed in a few relatively simple steps. A liquid composition which contains two solvents is mixed with a trichlorethylene-based phosphatizing bath sample of known weight and/or volume. One of these solvents is a good solvent for alkyl acid phosphates, as well as the other bath ingredients including trichlorethylene, but a relatively poor solvent for the phosphoric acid. The other of these solvents is a good solvent for the phosphoric acid, but a relatively poor solvent for the other bath ingredients (alkyl acid phosphates, trichlorethylene, etc.). The two solvents are immiscible and substantially insoluble in each other and sufiiciently different in specific gravity from each other so that two readily separable solution layers are produced when the solvents are mixed together.

Any combination of solvents having the above mentioned characteristics can be utilized. Typical solvents for the alkyl acid phosphates are generally alcohols. The preferred alcohols include n-pentanol, n-hexanol and nheptanol. In addition to these alcohols, secondary aliphatic alcohols, tertiary aliphatic alcohols and aryl alcohols which possess the above-defined characteristics can be used. The preferred alkyl acid phosphate solvent is npentanol. A good solvent for the phosphoric acid is water and is the preferred phosphoric acid solvent for this invention.

The amount of each of the solvents added to the bath sample can vary considerably but shoud be in sufficient amounts to dissolve the ingredients and form separate layers. Generally, the amount of solvent for the trichlorethylene and alkyl acid phosphates should be from 3 to 10 times by weight, and preferably about times by weight, greater than the weight of the bath sample. This is to insure suflicient dilution of the trichlorethylene and also to provide a sufficiently low specific gravity to give rapid separation from the other solvent-phosphoric acid layer. The amount of solvent for the phosphoric acid should be about the same as the amount of solvent for the other ingredients. Thus, a desirable weight ratio of each solvent to the phosphatizing bath sample is in the range of from about 3:1 to 10:1, and preferably 5:1.

After the solvents have been added to the phosphatizing bath sample, the resulting mixture is shaken vigorously to allow the bath ingredients to segregate to the respective solvents. The phosphoric in the original bath sample will segregate between the two solvents in such a way that substantially all of it is in the water phase; conversely, the trichlorethylene and alkyl acid phosphates will segregate substantially entirely into the organic solvent phase. Upon standing the mixture separates into two layers. The water layer, which is usually the lower layer, is separated from the organic solvent layer and titrated with a standard basic solution (e.g. sodium hydroxide solution) to a pH end point of 4.50, as typically measured with bromophenol blue indicator. The titer value is equivalent to the amount of free phosphoric acid (Active Acid value) in the phosphatizing bath sample.

The preferred procedure for measuring the free phosphoric acid of a typical, previously described trichlorethylene-based phosphatizing bath is as follows: A 30 gram sample is withdrawn from the trichlorethylene-based phosphatizing bath. This sample, together with 150 ml. of n-pentanol and ml. of distilled water are placed in a separatory funnel. The funnel is stoppered and shaken vigorously for one minute, pausing briefly at intervals to invert the funnel and to open the stop-cock to relieve any pressure build-up. The funnel is then placed in a ring stand, and the funnel contents are permitted to separate by gravity into two distinct layers. This usually requires 5 to 15 minutes. The stopper is then removed and the bottom layer containing water and phosphoric acid is drawn off into a separate container (e.g. a 500 ml. Erlenmeyer flask.). Six to ten drops of bromophenol blue indicator are added to the container; this turns the water solution to a yellow color. The contents of the container are then titrated using a burette containing 0.1 N Sodium hydroxide solution; the container is swirled vigorously. When the color of the contents in the container turns from yellow to a permanent blue, the burette (preferably calibrated in ml.) reading is recording as ml. of free phosphoric acid for a 30 gram sample of the phosphatizing bath. Of course, the amount of free phosphoric acid in the entire phosphatizing bath can be readily calculated from the known weight of free phosphoric acid per 30 grams of the bath.

As can be seen, the method of this invention for determining the amount of free phosphoric acid is rapid, accurate and does not require the use of an expensive pH meter. In addition, by taking this free phosphoric acid content and subtracting it from the empirical desired free phosphoric acid value, the amount of free phosphoric which is necessary to maintain the desired free phosphoric acid content can be calculated. This amount of phosphoric acid can be subsequently added to the bath to maintain the desired phosphoric acid content in the trichlorethylenebased phosphatizing bath.

By using the teachings of this invention, the free phosphoric acid content of a trichlorethylene-based phosphatizing bath can be readily calculated and adjusted to maintain a constant coating rate.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. A process for determining the free phosphoric acid content of a trichlorethylene-based phosphatizing bath comprising (a) obtaining a weighed bath sample, (b)

adding to the sample a liquid composition consisting essentially of two immiscible solvents, one of said solvents being a good solvent for alkyl acid phosphates and a relatively p001 solvent for phosphoric acid, the other of said solvents being a good solvent for phosphoric acid and a relatively poor solvent for alkyl acid phosphates, said two solvents being substantially insoluble and immiscible in each other and sufliciently different in specific gravity from each other so as to form two separate solution layers, (c) separating the two solution layers, and (d) titrating the solution layer containing the phosphoric acid with a standard basic solution to obtain a value which is the measure of free phosphoric :acid in said bath per weight of bath sample.

2. A process according to claim 1 wherein the phosphatizing bath comprises 72 to 98 percent by weight of trichlorethylene, 2 to 25 percent by weight of C to C aliphatic alcohol, and 0.1 to 3 percent by weight of orothphosphoric acid.

3. A process according to claim 1 wherein the weight 6 ratio of each solvent to bath sample is in the range of from about 3:1 to 10:1.

4. A process according to claim 1 wherein the solvent for alkyl acid phosphates is selected from the group consisting of n-pentanol, n-hexanol and n-heptanol and the solvent for phosphoric acid is water.

References Cited UNITED STATES PATENTS 4/1957 Copelin 1486.15 1/1963 Long, et a1.

891,390 4/1962 Great Britain.

MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner U.S. Cl. X.R. 23165, 312 

